The proposed research in this grant concerns the nervous systems of the leech and Aplysia. In the leech, we will attempt to demonstrate that 5-hydroxytryptamine is the neurotransmitter used by two large cells, the Retzius cells. We will attempt to trace the connections of these large cells. We will attempt to identify the terminals of the smaller 5-HT containing cells in this nervous system. In Aplysia, we will attempt to further our understanding of the control of the reproductive system in this animal by sensory-motor cells which appear to be specialized for sensory reception at one end, whereas the other end of the cell seems to be concerned with motor control of effector cells. We also will do further studies on individually identified axons. All these experiments are steps toward our long term goal of understanding the behavior of an organism in terms of the electrical activity and synaptic interconnections of individually identified neurons.